Gas turbines are commonly used for the generation of electrical power due to their compact size, high efficiency, and low capital cost. Gas turbines may be deployed in a combined cycle system that pairs a combustion turbine generator (“CTG”) with a heat recovery steam generation (“HRSG”). In a combined cycle system, the CTG produces electricity and then the exhaust from the CTG is used by the HRSG to generate steam that can be introduced to a stream turbine to generate additional electricity.
The CTG can use a compression chamber to compress air to a high pressure and temperature. The compressed air is then introduced into a compression chamber where a constant pressure combustion of the fuel occurs. The hot exhaust is then expanded across a turbine to let down the pressure and decrease the temperature. The work of the CTG is to spin an axle which is connected to an electrical generator, which produces alternating current. The hot exhaust from the gas turbine can be used to create steam in the RSG, and this steam can be used to spin a steam turbine generator, or STG, creating more electricity.
Molten carbonate fuel cells utilize hydrogen and/or other fuels to generate electricity. The hydrogen may be provided by reforming methane or other reformable fuels in a steam reformer that is upstream of the fuel cell or within the fuel cell. Reformable fuels can encompass hydrocarbonaceous materials that can be reacted with steam and/or oxygen at elevated temperature and/or pressure to produce a gaseous product that comprises hydrogen. Alternatively or additionally, fuel can be reformed in the anode cell in a molten carbonate fuel cell, which can be operated to create conditions that are suitable for reforming fuels in the anode. Alternately or additionally, the reforming can occur both externally and internally to the fuel cell.
Traditionally, molten carbonate fuel cells are operated to maximize electricity production per unit of fuel input, which may be referred to as the fuel cell's electrical efficiency. This maximization can be based on the fuel cell alone or in conjunction with another power generation system. In order to achieve increased electrical production and to manage the heat generation, fuel utilization within a fuel cell is typically maintained at 70% to 75%.
U.S. Patent Application Publication No. 2011/0111315 describes a system and process for operating fuel cell systems with substantial hydrogen content in the anode inlet stream. The technology in the '315 publication is concerned with providing enough fuel in the anode inlet so that sufficient fuel remains for the oxidation reaction as the fuel approaches the anode exit. To ensure adequate fuel, the '315 publication provides fuel with a high concentration of H2. The H2 not utilized in the oxidation reaction is recycled to the anode for use in the next pass. On a single pass basis, the H2 utilization may range from 10% to 30%. The '315 reference does not describe significant reforming within the anode, instead relying primarily on external reforming.
U.S. Patent Application Publication No. 2005/0123810 describes a system and method for co-production of hydrogen and electrical energy. The co-production system comprises a fuel cell and a separation unit, which is configured to receive the anode exhaust stream and separate hydrogen. A portion of the anode exhaust is also recycled to the anode inlet. The operating ranges given in the '810 publication appear to be based on a solid oxide fuel cell. Molten carbonate fuel cells are described as an alternative.
U.S. Patent Application Publication No. 2003/0008183 describes a system and method for co-production of hydrogen and electrical power. A fuel cell is mentioned as a general type of chemical converter for converting a hydrocarbon-type fuel to hydrogen. The fuel cell system also includes an external reformer and a high temperature fuel cell. An embodiment of the fuel cell system is described that has an electrical efficiency of about 45% and a chemical production rate of about 25% resulting in a system coproduction efficiency of about 70%. The '183 publication does not appear to describe the electrical efficiency of the fuel cell in isolation from the system.
An article in the Journal of Fuel Cell Science and Technology (G. Manzolini et. al., J. Fuel Cell Sci. and Tech., Vol. 9, February 2012) describes a power generation system that combines a combustion power generator with molten carbonate fuel cells. Various arrangements of fuel cells and operating parameters are described. The combustion output from the combustion generator is used in part as the input for the cathode of the fuel cell. One goal of the simulations in the Manzolini article is to use the MCFC to separate CO2 from the power generator's exhaust. The simulation described in the Manzolini article establishes a maximum outlet temperature of 660° C. and notes that the inlet temperature must be sufficiently cooler to account for the temperature increase across the fuel cell. The electrical efficiency (i.e., electricity generated/fuel input) for the MCFC fuel cell in a base model case is 50%. The electrical efficiency in a test model case, which is optimized for CO2 sequestration, is also 50%.
An article by Desideri et al. (Intl. J. of Hydrogen Energy, Vol. 37, 2012) describes a method for modeling the performance of a power generation system using a fuel cell for CO2 separation. Recirculation of anode exhaust to the anode inlet and the cathode exhaust to the cathode inlet are used to improve the performance of the fuel cell. The model parameters describe an MCFC electrical efficiency of 50.3%.